Get out in the Sun! The Glorious Sun! It heals the heart and soul. I had a "winter baby" and so, had the winter blues due to lack of sunlight exposure on top of the postpartum depression. (I think I was a tree or flower in a past life, because I feel I am lost without the sun!)
I have had issues with being sad in the winter before, but not in the two winters before my baby was born. I wondered why, and I figured it out: last year I was outside everyday as a part of my job, and the year before, I went to Vegas and Mexico. (It was a great year for me!)
So, get out! Go for a walk, even when the sun isn't shining. Look for nature paths near your home. The sun plus the exercise will do wonders for your mood!!